The present invention relates to systems for maintaining extremely low gas pressures during execution of industrial and scientific processes, particularly in regions which are continuously receiving a fresh supply of gas.
Many types of industrial and scientific processes are performed in a region which is evacuated to a very low pressure, of the order of several milliTorr (mT). Processes of this type include deposition and etching operations performed on semiconductor wafers with the aid of a plasma. In systems for carrying out such processes, a plasma is generated in a processing region which contains a processing gas maintained at a low pressure in the range of 1-100 mT, and frequently less than 10 mT. The gas will be ionized in the plasma and the resulting ions can be accelerated toward the wafer by suitable electric fields. During the course of the process, processing gas must be pumped out of the region at a high rate with a minimum of contamination by foreign materials, such as oil, that may be contained in the pumping equipment, and materials resulting from the processing itself, while fresh processing gas is supplied to the region.
The conventional technology employed to create low pressure levels of the order indicated above generally utilizes either one of the following two fundamental mechanisms: (1) increasing the momentum of gas molecules (hereinafter, references to xe2x80x9cmoleculesxe2x80x9d will be understood to encompass both atoms and molecules in those contexts where reference to both types of particles would be more technically correct) in a preferred direction and exhausting the gas through a valve or a baffle structure which inhibits reverse flow of gas; or (2) condensing the gas on specially prepared surfaces. Mechanism (1) is usually implemented with some type of piston, blower, or rapidly moving vanes which impart directed momentum to the gas by employing rapidly moving mechanical structures or streams of pumping molecules, such as molecules of mercury or readily condensable pumping oils. Mechanism (2) is commonly used in systems with low to moderate throughput requirements.
Turbomolecular pumps utilize mechanism (1) and are provided with rapidly spinning discs which impart directed momentum to gas molecules by colliding with those molecules. This mechanism is most effective for gas pressures which are sufficiently low that the mean-free-path of the molecules is longer than the dimensions of the pumping structures.
To establish low pump inlet pressures, of the order of 1-100 mT, employed in industrial plasma processes, it is presently the nearly universal practice to employ turbomolecular pumps as the first stage of a compound pumping system intended to pump large quantities of processing gas.
It has been found that the quality of processing operations of the type described above, and thus the quality of the finished semiconductor device, is dependent in large measure on the purity and composition of the processing gas and that these parameters can best be controlled if the flow rate of fresh gas into the processing enclosure is relatively high. The quality of the results produced by plasma assisted etching and deposition processes could be significantly enhanced if the gas throughput, or rate of flow of gas into and out of the processing region, could be increased to a level between 3 and 5 times that presently utilized.
Although there are currently available high speed turbomolecular pumps which can achieve throughputs of the order of 5500 liters per second, at low inlet pressures, the highest capacity pumps that are currently available are also extremely expensive, as well as being less reliable than smaller pumps.
Moreover, even a throughput of 5500 liters per second has been found to be less than the optimum value for performing processes on wafers having a diameter of 200 mm, while achievement of optimum processing results on larger diameter wafers would require even higher gas throughput. In general terms, the gas throughput required to achieve a certain processing result in terms of quality is proportional to the area of the substrate.
In addition, effective control of gas flow must allow for gas species that have a tendency to become attached to solid surfaces within the system. Such species include, for example, carbon compounds that are polymerized either by electrons or protons in the plasma. A plasma electron or proton flux can easily affix such materials to solid surfaces. Such materials may be subsequently released from the surfaces, perhaps in a modified form. The quality of any of the plasma assisted processes of the type described above is dependent on the extent to which polymerized or otherwise modified materials can be prevented from being deposited on the substrate surface and this, in turn, depends on the extent to which such materials can be prevented from forming and/or remaining in the processing region.
Gas molecules which remain in the processing region for any significant time can be deposited on the substrate in a chemical form which is resistant to subsequent etching processes. As a result, these molecules will form defects on the substrate surface.
In view of the possible occurrence of such phenomena, it is apparent that the shorter the residence time of gas molecules in the processing region, the higher will be the quality of the product resulting from a series of etching and/or deposition processes.
In addition to the vacuum pumping technologies that have been used in connection with processing operations of the type described above, pumps using a plasma as an active element have been proposed. Plasma vacuum pumps would be capable of pumping a variety of gasses, including hydrogen and helium, with relatively high efficiencies, and are relatively immune to damage by solid or corrosive materials.
The operation of plasma vacuum pumps involves transforming three-dimensional flow of a neutral gas into one-dimensional flow guided by a magnetized plasma which may be magnetically compressed and guided through suitable baffle structures. Momentum can be imparted to the plasma as a result of various electromagnetic interactions and can be imparted to the neutral gas through collisions between molecules of the neutral gas and moving ions which have been accelerated and have greater momentum than background gas.
However, the potential benefits of using plasma vacuum pumps in plasma processing systems has not heretofore been realized to any significant extent. In particular, no solution has been proposed which combines efficient plasma generation with the creation of magnetic fields compatible with the plasma processing operation to be performed and suitable for channeling the plasma, as well as with a suitable mechanism for effecting pumping at pressures in the range which is of importance in such plasma processing operations. In this connection, there have been no proposals which take into account the difficulties created by the ability of a plasma to shield its interior region from low frequency external electric fields.
The possibility of employing a plasma vacuum pumping in plasma processing systems has been described, for example, in U.S. Pat. No. 4,641,060, which is issued to Dandl on Feb. 3, 1987. This patent discloses a plasma vacuum pump which does not employ any moving mechanical parts and which is capable of producing high pumping rates at gas pressures of less than 1 mT. A primary mechanism underlying this plasma vacuum pump is a magnetically guided flow of plasma ions and electrons through simple tubular baffle structures that restrict the flow of neutral gas molecules back into the region which is to be maintained at a low pressure. The pump disclosed in this patent appears capable of functioning effectively with magnetized plasmas at pressures below an upper limit determined by the spontaneous formation of electrostatic potentials that block the flow of plasma ions. xe2x80x9cMagnetized plasmasxe2x80x9d as used herein is a plasma in which the electron flow is magnetized, i.e. the electrons circulate around the magnetic field lines. While this form of plasma vacuum pump may prove suitable for some low pressure, magnetically confined plasma applications, it does not appear to be particularly suitable for typical industrial plasma processing systems.
It is an object of the present invention to pump ions out of a low pressure region at a high rate.
Another object of the invention is to provide a pumping system capable of achieving high pumping rates at a low cost.
A further object of the invention is to improve plasma vacuum pumping by preventing build-up of positive electrostatic potentials that would otherwise reduce the flow of ions through outlet passages.
A still further object of the invention is to permit electrical control of the rate at which ions are pumped.
The above and other objects are achieved according to the invention, by the provision of a novel plasma vacuum pumping method and pumping cell for pumping ions from a first region, the ions possibly being generated by a plasma in the first region, to a second region when the plasma pumping cell is interposed between those regions. The plasma pumping cell is composed of: a partition member positionable between the first region and the second region, the partition member having a through opening defining a conduit; a plurality of magnets positioned relative to the conduit in a manner to provide lines of magnetic force that extend along the conduit; a source of free electrons in communication with the conduit; and an electric potential source disposed relative to the conduit to create an electrostatic field which accelerates ions from the conduit to the second region.
The magnetic field produced by the magnets essentially influences the radial distribution of ions in the conduit and acts to trap electrons in the conduit. The trapped electrons act to prevent a positive space charge from developing within the conduit and give rise to an electrostatic field which accelerates positive ions from the first region into the conduit. While passing through the conduit, a certain proportion of the positive ions will combine with electrons to form neutral molecules. These neutral molecules will be carried by momentum into the second region. Ions which have not combined with ions will be carried into the second chamber by momentum and the electrostatic field produced by the potential source.
The plasma in the first region ionizes a gas which has been introduced into that region, producing electrons that are magnetized and ions that are typically employed to carry out a process and that are to be pumped out of the first region so that a fresh supply of ions will be maintained in that region. The effect of the plasma on the electrons is to create a space charge that affects ion motion. According to one novel feature of the invention, the ions in the first region are given some directed energy, that aids the pumping action, by the plasma.
According to a further novel aspect of the present invention, the pumping cell or cells are positioned relative to the processing region in a manner to increase the efficiency with which ions can be pumped out of that region. Preferably, the cell or cells are disposed close to the plasma in the processing region so that use can be made of the highest possible plasma density to improve pumping efficiency. This configuration allows the average time that gas molecules remain in the processing region to be made extremely short.
A plasma vacuum pump composed of a plurality of cells according to the present invention may be disposed in a processing chamber which contains a substrate to be processed. It is desirable to form the plurality of cells in a partition wall having an area which is larger than that the surface area of the substrate in order to increase the pumping rate and thus reduce the residence time of gas molecules in the chamber. However, this should be accomplished without significantly increasing the volume of the processing chamber since the residence time of gas in the processing chamber will increase as the chamber volume increases.
When the ions to be pumped have been generated in a plasma, ions adjacent the boundary of the plasma region will be influenced by the plasma sheath and ions in the area of a pumping cell will experience some pre-sheath motion toward the cell.
These ions will then be attracted into the cell conduit by the electrostatic field created by electrons trapped in the conduit. The intensity of this electrostatic field is determined essentially by the ambient ion density and electron density and the electric field produced by the electric potential source. The electrostatic field extends through the conduit and acts to accelerate ions from the first region, near the partition member, into the conduit.
In addition, momentum will be imparted to unionized gas molecules in the chamber by ions of gas molecules that have just entered the chamber and have a low, finite level of directed energy. Collisions between these ions and neutral species of exactly the same molecular structure and weight results in a very efficient, or resonant, transfer of energy. The efficiency of the transfer is inversely proportional to the speed of the faster one of the two colliding particles.
Gas molecules which are to be pumped need not be transported through any transitional structure in order to be removed from the system. In fact, processing gas molecules which have been injected into the processing region, or chamber, and which have not yet been ionized, will naturally travel in one of the following two possible ways:
(a) the more probable behavior of the molecules will be to bounce off of the surface of the substrate being processed and to be reflected in a cosine distribution about a normal to the substrate surface. The breadth of this distribution depends upon the fine surface features of the substrate;
(b) otherwise, the molecules may be adsorbed on the surface of the substrate and will then be re-emitted sometime later, possibly after having been converted to a different species having a different molecular weight. Such re-emission is also associated with a cosine distribution about the normal to the substrate surface.
The above-mentioned cosine-distributions describe the pattern in which molecules are reflected or re-emitted in different directions from the substrate surface. The intensity, or quantity, of molecules emitted in each direction is proportional to the cosine of the angle formed between the direction and the substrate surface, which means that the intensity, or quantity, of emitted electrons will be greatest in the direction Perpendicular to the substrate surface.
Control of gas flows within the processing chamber is enhanced if gas is introduced into the chamber in such a manner as to be propelled directly toward the substrate surface and if most molecules which have bounced off of the substrate surface are removed from tire processing chamber by the vacuum pump after the first bounce off of the substrate surface. A simple calculation of molecular speed ndicates that effective use of the kinetic energy of molecules to be removed requires that the number of bounces, or reflections, experienced by the molecules be minimized. To the extent that gas molecules can be pumped out of the processing chamber immediately after having bounced off the substrate surface, collisions with surfaces within the processing chamber can be minimized. Such minimization is advantageous because molecules which strike such surfaces may become attached thereto and may subsequently be returned to the substrate surface in an altered form. Therefore, rapid removal of gas molecules allows the process being performed on the substrate to be controlled in a superior manner.